Phosphorus tribromide

Phosphorus tribromide is a colourless liquid with the formula PBr₃. It fumes in air due to hydrolysis and has a penetrating odour. It is widely used in the laboratory for the conversion of alcohols to alkyl bromides.

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Chemical properties

Phosphorus tribromide, like PCl₃ and PF₃, has both properties of a Lewis base and a Lewis acid. For example, with a Lewis acid such as boron tribromide it forms stable 1:1 adducts such as Br₃B-PBr₃. At the same time PBr₃ can react as an electrophile or Lewis acid in many of its reactions, for example with amines.

The most important reaction of PBr₃ is with alcohols, where it replaces an OH group with a bromine atom to produce an alkyl bromide. Note that all three bromines can be transferred.

PBr₃ + 3 ROH → 3 RBr + HP(O)(OH)₂

The mechanism (shown for a primary alcohol) involves initial activation of the alcohol oxygen by the electrophilic phosphorus (to form a good leaving group), followed by an S_N2 substitution at the alcohol carbon.

Because of the S_N2 substitution step, the reaction generally works well for primary and secondary alcohols, but fails for tertiary alcohols. If the reacting carbon centre is chiral, the reaction usually occurs with inversion of configuration at the alcohol alpha carbon, as is usual with an S_N2 reaction.

In a similar reaction, PBr₃ also converts carboxylic acids to acyl bromides.

PBr₃ + 3 RCOOH → 3 RCOBr + HP(O)(OH)₂

PBr₃ is a reasonably strong reducing agent, and the oxidation of PBr₃ with oxygen gas is more vigorous than seen with PCl₃. It gives an explosive reaction that forms P₂O₅ and Br₂.

Preparation

PBr₃ is prepared by treating phosphorus with bromine, using PBr₃ itself as the solvent (white phosphorus is soluble in PBr₃). An excess of phosphorus is used in order to prevent formation of PBr₅.

P₄ + 6 Br₂ → 4 PBr₃

Uses

The main use for phosphorus tribromide is for conversion of primary or secondary alcohols to alkyl bromides,^[1] as described above. PBr₃ usually gives higher yields than hydrobromic acid, and it avoids problems of carbocation rearrangement- for example even neopentyl bromide can be made from the alcohol in 60% yield^[2].

Another use for PBr₃ is as a catalyst for the α-bromination of carboxylic acids. Although acyl bromides are rarely made in comparison with acyl chlorides, they are used as intermediates in Hell-Volhard-Zelinsky halogenation].^[3] Initially PBr₃ reacts with the carboxylic acid to form the acyl bromide, which is more reactive towards bromination. The overall process can be represented as

On a commercial scale, phosphorus tribromide is used in the manufacture of pharmaceuticals such as alprazolam, methohexital and fenoprofen. It is also a potent fire suppression agent marketed under the name PhostrEx [4].

Precautions

PBr₃ evolves corrosive HBr, is toxic, and reacts violently with water and alcohols.

In reactions that produce phosphorous acid as a by-product, when working up by distillation be aware that this can decompose above about 160 °C to give phosphine which can cause explosions in contact with air.^[5]

References

1. ^  N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, UK, 1997.
2. ^  Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
3. ^  J. March, Advanced Organic Chemistry, 4th ed., p. 723, Wiley, New York, 1992.
4. ^  The Merck Index, 7th edition, Merck & Co, Rahway, New Jersey, USA, 1960.
5. ^  R. R. Holmes, Journal of Inorganic and Nuclear Chemistry 12, 266-275 (1960).
6. ^  L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 477, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.
7. ^  George C. Harrison, H. Diehl, in Organic Syntheses Collective Volume 3, p 370, Wiley, New York, 1955.
8. ^  L. G. Wade, Jr., Organic Chemistry, 6th ed., p. 1051, Pearson/Prentice Hall, Upper Saddle River, New Jersey, USA, 2005.